Stephen,
As Scott pointed out, the engine size will play a large part in the cost, generally the bigger the engine....the higher the price. The other player is the options (I took out condition since you stated you wanted 'turn key'). This will be a factor since highly optioned cars are more sought after. But engine size is the biggest factor and the 289 engine is widely available. I would say you can purchase a very nice GT/CS with a 289 and options like power steering and A/C for between $15,000 and $20,000.